Polymerization processes are exothermic and require cooling to remove the heat of reaction. This cooling is typically accomplished by installing a cooler, typically referred to as a cycle cooler, in the process cycle. Production capacity over periods of operation can be adversely affected by a gradual decrease in the cooling capacity of a cycle cooler. The rate of loss in cooling capacity varies with processes, catalyst types, and unit configurations. The amount of loss that can be tolerated by the process also varies with catalyst type, reactor configurations, and process economics. Gas phase processes typically require very large cycle coolers, and are particularly sensitive to losses of cooler capacity. Some processes, such as processes using metallocene-type catalyst systems, are particularly sensitive to changes in cooling capacity.
The mechanisms for losses in cooling capacity vary. For example, particles of catalyst and/or resin may be carried into the cycle cooler and deposited on the process side (typically the inside) of the cooler tubes. These deposits not only restrict the flow of fluid, but also reduce the transfer of heat from the process fluid into the coolant. In liquid phase processes, some of the polymer may dissolve in the reactor diluent and redeposit on the metal heat exchanger surfaces.
Process operability problems beyond cooling capacity are also a major issue for polymer processes. For example, the tendency for a continuous gas phase or slurry phase process to form a blockage and/or sheet is a concern. Sheeting and/or static generation in a continuous gas phase process can lead to the ineffective operation of various reactor systems resulting in early reactor shutdown. In a continuous slurry process, polymer particles accumulating on the walls of the reactor, which act as a heat transfer surface, can result in operability problems. These polymer particles can continue to polymerize on the walls and can result in a premature reactor shutdown. These sheeting problems are referenced in some prior art as “fouling” of the system. “Fouling” in this context refers to physical blockage of material flow in or through the reactor vessel by sheets and agglomerates. This is a different problem from the term “fouling” of a process cooler. When the term “fouling” is applied to process coolers, it typically refers to the gradual loss of cooling capacity as a result of buildup of material on or in the cooler tubes. Thus, methods of preventing “fouling” in reactor vessels are typically not applicable to the prevention of “fouling” (loss of cooling capacity) in cycle coolers.
Advances in polymerization processes have addressed many reactor continuity problems. For example, U.S. Pat. No. 6,608,153, and 6,630,984 are directed to adding a carboxylate metal salt into the reaction system to reduce static or sheeting and thus improve process continuity. In addition, U.S. Pat. Nos. 6,803,430, 6,946,530, 6,894,127, 6,646,074, 6,639,028, 6,562,924, 6,887,957, 6,180,738, and U.S. Patent Publ. No. 20040087743 discuss polymerization using various additives and techniques to reduce sheeting and improve reactor continuity; U.S. Pat. Nos. 5,264,505, and 5,728,782 discuss methods to reduce cooler or other fouling in processes; CHEMECA 82 [Eighty-Two]: Resour. Dev. Eighties, Aust. Chem. Eng. Conf., 10th (1982), 304-7. Inst. Eng. Aust.: Barton, Australia discusses the addition of Polyox WSR-301 to reduce the friction in plate heat exchangers; and U.S. Pat. Nos. 5,166,279 and 6,165,418 and U.S. Patent Publ. No. 20040225098, discuss polymer processes.
For example, antistatic agents, such as, carboxylate acids, and/or metal salts have been used to reduce the amount of sheet and agglomerate formation in the polymerization reactor. In particular, EP-A1 0 453 116 published Oct. 23, 1991 discusses the introduction of antistatic agents to the reactor for reducing sheeting and agglomerate formation; U.S. Pat. No. 5,026,795 discusses the addition of an antistatic agent with a liquid carrier to the polymerization zone in the reactor; U.S. Pat. No. 5,410,002 discusses using a conventional Ziegler-Natta titanium/magnesium supported catalyst system where a selection of antistatic agents are added directly to the reactor to reduce sheeting and agglomeration; U.S. Pat. Nos. 5,034,480 and 5,034,481 discuss a reaction product of a conventional Ziegler-Natta titanium catalyst with an antistat to produce ultrahigh molecular weight ethylene polymers; U.S. Pat. No. 3,082,198 discusses introducing an amount of a carboxylic acid dependent on the quantity of water in a process for polymerizing ethylene using a titanium/aluminum organometallic catalysts in a hydrocarbon liquid medium; U.S. Pat. No. 3,919,185 describes a slurry process using a nonpolar hydrocarbon diluent using a conventional Ziegler-Natta-type or Phillips-type catalyst and a polyvalent metal salt of an organic acid having a molecular weight of at least 300; and PCT Publication WO 97/46599 published Dec. 11, 1997 discusses feeding an unsupported, soluble metallocene-type catalyst system and injecting antifoulants or antistatic agents into the reactor.
Others have discussed modifying the catalyst system to improve process operability. Examples of these include: WO 96/11961 published Apr. 26, 1996 discusses as a component of a supported catalyst system and antistatic agent for reducing sheeting in a gas, slurry or liquid pool polymerization process; and EP-A2-811 638 discusses using a metallocene catalyst and an activating cocatalyst in a polymerization process in the presence of a nitrogen containing antistatic agent.
Background references include: WO 1998/12231, WO 2000/02931, WO 2001/44323, WO 2004/103099; EP 1 576 857; CA 2264463; U.S. Pat. Nos. 6,939,980, 6,805,801, 6,372,868, 4,501,319, and 3,431,279; U.S. Patent Application Publication No. 2005/267267.
The collective effect of process improvements has resulted in the capability to operate polymerization processes for extended periods of time without shutting down the processes due to unplanned process interruptions or maintenance of the system. However, cycle coolers continue to lose cooling capability over time. Thus, at some point a unit must be shutdown and the cycle cooler must be cleaned to restore the original cooling capacity of the system. Valuable production operating time is lost while the unit is shut down for cooler cleaning. Furthermore, the costs and/or amount of downtime required to clean a cycle cooler can be significant.
Maintaining cooling capacity (prevention of losses in cooling capacity) is typically addressed by various mechanical designs. For example, a cycle cooler will be designed with a specific material, velocity, or flow pattern to minimize plugging and the resulting loss of cooling. Process operating conditions can also be controlled to prevent cycle cooler pluggage. For example, the temperature of the cooling medium may be controlled to control tube wall temperatures in a desired range. However, the past endeavors have failed to improve cycle cooler life to keep up with current extended operating capabilities. With the extended operating time, cycle cooler pluggage and loss of cooling capacity remains a concern in polymerization processes.
Thus, it would be advantageous to have a polymerization process capable of operating continuously while maintaining peak cooling capacity. In other words, it would also be highly beneficial to have a continuously operating polymerization process that does not lose a significant amount of cooling capacity over time.